CCDs are used in a variety of applications, such as spectrometry, imaging, digital memories, various logic functions, and signal processing, for accumulating, storing and/or transferring electrical charges. The basis of a typical CCD is the dynamic storage and withdrawal of electrical charge in a series of metal-oxide-semiconductor ("MOS") capacitors. To define the MOS capacitors, the CCD can include a number of metal gate electrodes positioned adjacent to an oxide-semiconductor substrate. In response to an applied electrical potential, a depletion region is formed under a gate electrode, and the surface potential increases considerably under a gate electrode. The potential well is used for accumulation and/or storage of electrical charge. The substrate further includes a number of channel stops for laterally confining the stored electrical charge(s) in a number of channels, each containing a plurality of potential wells.
In known CCDs, to transfer electrical charge(s) within each channel, adjacent gate electrodes are cyclically biased at predetermined intervals with different electrical potentials to create adjacent potential wells of differing depths. The electrical charges move along the channels from shallower potential wells beneath gate electrodes having a low electrical potential to deeper potential wells beneath gate electrodes having a high electrical potential. By alternately biasing adjacent gate electrodes at timed high and low electrical potentials, electrical charges can be sequentially shifted down the channels for processing and/or collection.
In light dispersive spectrometry applications, electrical charges are accumulated in potential wells during an integration period. The charge results from photons of radiation contacting a radiation-sensitive substrate. The amount of charge accumulated in each potential well is a function of the incident radiation intensity and the duration of exposure. Typically, the electrical charges accumulated in each channel are sequentially combined during shifting in a potential well in a process known as binning.
As will be appreciated, signal losses can occur in a CCD in light dispersive spectrometry applications. By way of example, a portion of the radiation can be absorbed by the relatively thick and wide polysilicon electrodes used in charge shifting. The polysilicon can act as an absorption filter preventing light in the visible, UV and near IR from reaching the channels. This absorption reduces the quantum efficiency of the CCD. Spurious charge can be generated when the phase clock drives are driven in and out of inversion (i.e., charge generation) during charge shifting (i.e., charge transfer from pixel to pixel). A fraction of an electrical charge can be generated with each charge shift between pixels. It is an obvious design consideration to minimize CCD noise to maximize the accuracy, responsiveness, and/or reliability of the CCD.